Multi-carrier communication systems for conducting communication by dividing transmission information into a plurality of frequency bands hereinafter called “sub-carriers” have been employed with the progress of wide band wireless communication technology. Among the multi-carrier communication systems, an OFDM (Orthogonal Frequency Division Multiplexing) system can eliminate the necessity for a guard band between sub-carriers by using a plurality of frequencies having an orthogonal relationship within a symbol time range while improving resistance to a delay wave by narrowing a bandwidth per sub-carrier and can keep frequency utilization efficiency. Therefore, the OFDM system has been employed in various kinds of systems as typified by digital television broadcasting e.g. ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) and wireless LAN such as IEEE802.11a.
In these communication systems, a pilot signal having a fixed pattern is inserted into a transmission signal and synchronous detection is made at the time of reception by using the pilot signal as reference phase and reference amplitude. As for the insertion interval, the pilot signal must be inserted in an interval sufficiently faster (shorter) than a time-wise fluctuation speed of a propagation route in a time direction and detection must be made upon reception by updating the reference phase. As for a frequency direction, on the other hand, the OFDM system executes collectively a reception signal processing for those signals which are received through a plurality of propagation channels having mutually different delay times. Since the reception signal phase is different depending on the frequency of the signal, pilot signals that operate as reference signals with a frequency interval sufficiently smaller than the differences of the reception signal amplitude and phase resulting from the frequency must be inserted.
The degree of fluctuation of the amplitude and the phase in the frequency direction is greater with greater dispersion of the delay time of the delay wave. Therefore, those systems which assume an outdoor use insert the pilot signals into a greater number of sub-carriers such as digital television broadcasting than in those systems which assume an indoor use such as wireless LAN. When the pilot signals are inserted into a large number of sub-carriers in this way, a configuration in which the pilot signals are arranged in symbols, called “scattered pilot symbols”, selected discretely in both time and frequency directions are ordinarily employed to avoid excessive dropping of data transmission efficiency.
FIG. 2 shows an example of the arrangement of the data symbols and the scattered pilot symbols on the time-frequency axis for ISDB-T that is described in Association of Radio Industries and Businesses, “Transmission System for Digital Terrestrial Television Broadcasting”, ARIB std-B31 version 1.5, July 2003, 3.12.2, OFDM-segment Configuration for the Synchronous Modulation (non-patent document 1).
FIG. 2 is a schematic view in which the abscissa represents the carrier number, that is, the frequency axis, and the ordinate does the OFDM symbol number or the time axis. Each box of a rectangular shape represents one modulation symbol such as QPSK and 16QAM. Numeral 121 in the drawing denotes the data symbol used for the communication of information and SP denoted by reference numeral 122 represents the pilot symbol used as the reference signal. To secure resolution in both time and frequency directions with a smaller number of pilot symbols, the pilot symbols 122 are arranged discretely on the time and frequency axes. From this arrangement, the pilot symbols are called “scattered pilot symbols”. Suffixes Si, j, etc, put to the data symbols 121 represent the data symbol that is ith in the frequency direction and jth in the time direction. The number in the frequency direction is the symbol number other than the scattered pilot symbols as shown in FIG. 2. Therefore, the number of data symbols arranged at a certain time is the balance obtained by subtracting the number of pilot sub-carriers from the total number of sub-carriers.
In the field of mobile communication, on the other hand, communications are made in one wireless band by multiplexing signals from a plurality of users having different requirements for communication quality, communication speed, etc, and a plurality of channels. As means for multiplexing and mapping in this case, a system has been examined which gathers one or a plurality of sub-carriers or one of a plurality of time symbols into one resource block, divides a channel and allocates the divided channels to the resource blocks.
For example, 3rd Generation Partnership Project, “Technical Specification Group Radio Access Network: Physical Layer Aspects for Evolved UTRA (Release 7)”, 3GPP TR 25.814 V1.2.0, February 2006, 7.1.1.2 Multiplexing including Reference—Signal Structure (non-patent document 2) describes a method that handles about 25 sub-carriers in the frequency direction and about 6 to about 7 symbols in the time direction as one resource block.